An image sensor is an apparatus for receiving light, i.e., photons, generated by or reflected from an object, and for generating digital image data. The manufacture of one particular type of image sensor may incorporate conventional complementary metal-oxide-semiconductor (CMOS) fabrication technology. Accordingly, such an image sensor is often referred to as a CMOS image sensor (CIS).
Generally, a CMOS image sensor includes a light sensing region for converting photon energy received from an object to an electrical signal (such as current) and a peripheral circuit region for processing and sending the electrical signal to another device for further processing. A photodiode is formed in the light sensing region, and transistors or other devices may be formed in the peripheral circuit region, thereby forming a semiconductor structure. An interconnect structure including a plurality of insulating layers and metal lines is formed over the semiconductor structure to interconnect the photodiode, transistors and other devices.
However, the design of the interconnect structure is typically based on desired electrical characteristics and device reliability rather than on optical characteristics. As a result, optical transmittance may be degraded by optical reflections at the interfaces between the layers of the semiconductor and interconnect structures.
For example, silicide layers are often employed to improve the electrical coupling of interconnected devices. However, silicides are optically opaque and exhibit excessive junction leakage. Therefore, a shallow-trench-isolation (STI) element may be formed over the photodiode, so that the photodiode is effectively masked during the silicidation process. However, the abrupt transition from the relatively low refractive index silicon dioxide layer to the relatively high refractive index silicon layer may cause high reflection of the optical energy.
Continued scaling of semiconductor devices has introduced the use of borderless contacts (see, e.g., U.S. Pat. No. 6,444,566, to Tsai, et al., commonly assigned herewith and incorporated herein by reference). Borderless contacts require the formation of a buffer layer, typically comprising SiN or SiON, between the STI and the overlying silicon dioxide layer. The borderless contact buffer layer once again introduces optical problems, causing destructive interference attributable to the varying refractive indices of the materials formed over the photodiode. That is, the silicon substrate in which the photodiode is formed has a relatively high refractive index, the STI element formed over the photodiode has a relatively low refractive index, the SiON buffer layer formed over the STI element has a medium refractive index, and the silicon dioxide layer formed over the buffer layer has a relatively low refractive index. Such variation in refractive indices through which impinging light must propagate for effective operation of the photodiode will cause destructive interference of the optical signal, especially for signals having wavelengths approaching the thickness of the STI element.
Accordingly, what is needed in the art is a photodiode device that addresses the problems discussed above.